In the first five parts of this series, I discussed various 2-compartment models of estimating body composition. These models divide the body into fat and fat-free mass. Now let's talk about a common 3-compartment model: dual-energy x-ray absorptiometry (DEXA).

DEXA

DEXA, once used for determining bone density, evolved into a technique for also estimating body composition. DEXA represents a 3-compartment model for estimating body composition, because it can divide the body into 3 compartments: fat, bone mineral, and all other fat-free mass that does not include bone. Thus, unlike 2-compartment models, DEXA is not subject to errors caused by variations in bone density among different ethnicities. However, there are other sources of error which I will discuss in this article.

DEXA offers a number of advantages. Improved technology has dramatically decreased scan times (years ago, a DEXA scan would take 20-25 minutes; now scans can take 5-10 minutes), so the method is convenient and non-invasive. DEXA can provide bone density estimates, and can provide regional estimates of body composition (meaning it can estimate the body composition of individual parts of your body). DEXA works by measuring your body's absorbance of x-rays at two different energies. Fat, bone mineral, and fat-free soft tissue have different absorption properties. Thus, we can get estimates of your body composition by scanning your entire body.

DEXA Sources of Error

Like all other body fat estimation techniques, DEXA has numerous sources of error. There can be inconsistent results between different machines from different manufacturers, and even different results between machines from the same manufacturer. Software upgrades can change the algorithms that the device uses to calculate body composition. Different hardware and software configurations can result in different interpolations of soft tissue over bone, and different treatment of pixels of which a small portion is bone. The type of X-ray beam (fan beam or pencil beam) can also be a source of error; DEXA machines with fan beams can suffer from beam magnification (also known as parallax error). A final source of error is the same error that all 2-compartment models also have: the hydration of fat-free mass. In fact, a 5% variation in fat-free mass hydration can change your DEXA-determined body fat percentage by nearly 3%. This can be a problem when comparing different ethnicities or body types where fat-free mass hydration can vary. It can also be a problem when trying to measure change over time.

There are also issues when trying to use DEXA for tracking change over time. In one study, researchers wrapped lard around the legs of subjects to simulate weight gain. However, this changed the apparent bone mineral content, questioning the ability of DEXA to accurately assess body composition during weight change. In another study on bodybuilders, DEXA was reasonably accurate when looking at the group change in body fat percentage, but the individual error rate was as high as 4%. Thus, you could decrease your body fat percentage by 4% yet DEXA might show no change, or DEXA might show a 4% change when you didn't really have a change. Another study showed DEXA to overestimate decreases in percent body fat, while it underestimated increases in percent body fat. In fact, the overestimations got as high as 5%. Finally, here's a chart from a study that compared changes in body fat over time using DEXA to using a 4-compartment model:

Change in % body fat, comparing DEXA to a 4-compartment model

The X-axis of this chart shows the change in % body fat for DEXA, while the Y-axis shows the change for the 4-compartment model. You can see there was very little agreement between the methods. For example, one person showed an increase in body fat percentage by 5% with the 4-compartment model, but a decrease by 5% according to DEXA. Another person had nearly a 10% loss of body fat according to the 4-compartment model, but only a 3% decrease according to DEXA. These results question the reliability of DEXA for measuring change over time in individuals.

DEXA: The Verdict

Despite the fact that DEXA represents a 3-compartment model, its error rates are no better than hydrostatic weighing, and in some cases is worse. Like other techniques, DEXA does well when looking at group averages, but not so well when looking at individuals. Individual error rates tend to hover around 5%, although some studies have shown error rates as high as 10%. When looking at change over time in individuals, error rates have hovered around 5% in some research, although other research has indicated DEXA to perform much more poorly. For these reasons, I do not recommend DEXA for tracking change over time in individuals. If you do use DEXA for tracking change over time, I recommend very long time periods between measurements (a minimum of 3-6 months), as you will need a minimum of a 5% change in body fat to reliably detect a true change in body fat in most people.

That sums it up for the most widely available techniques for determining body composition. In the final part next week, I will discuss the practical application of everything you have learned in this series.

I have performed research on the Body Metrix device and compared into to the BodPod and Tanita BIA Machine. They were highly correlated. To my knowledge there is no articles comparing ultrasound to dexa yet. My thesis is a cross validation of the BodPod, Body Metrix Ultrasound, DEXA, Skinfolds, BioSpace BIA Machine, and Anthropometrics. My paper is currently pending publication for the first research via the Body Metrix, Bodpod and Bia. Let me know if you have any questions. Thanks, Kelly

I am healthcare professional working closely with children. I am looking for a medical equipment thats safe and accurate for assessing obesity status / levels in children. Idea is to identify out of range cases at an early stage and dispense appropriate support promptly.

Must admit its a daunting task to identify the right equipment thats fit for the purpose.

I wish I read this before getting a DEXA scan for body composition. I’ve always done hydrostatic testing and I hover around 14% BF. I read that the DEXA scan was more accurate than the dunk tank so I thought I’d give it a go. I did the DEXA test right after the holidays and figured with a little holiday padding I’d probably be more like 16% right now, but the scan results suggested I am 24% body fat with 21 lbs less lean mass than was reported after my last hydrostatic test. I was planning to do a dunk test on the same day so I could compare, but the facility was not available so I will do that before long. My coach says there is no way I could be 24% BF, and there is no way I could have lost 21 lbs of lean mass.

Full disclosure…I own a body fat testing facility in Denver and we use a DEXA scan.

Yes results can vary as they can in all measurements with all methods. We almost always see higher body fat percentages with a DEXA than with hydrostatic weighing (underwater). Frankly the Bod Pod measurements are all over the place, so it is out of the discussion as far as I’m concerned.

All the testing methods have pluses and minuses. By far the DEXA has fewer negatives, assuming the machines are calibrated and software updated. I had tests done at a “regular” bone doctor and no way in H E double hockey sticks was my bf at the 11% it showed (flat belly, but no visible abs).

The biggest problem with a DEXA is expectations. Many women assume that 24% is horrible…when in fact that is a very fit (or genetically blessed) woman. Men also wrongly assume they are 16% when they have a belly that over hangs their belt.

We see variations in lean mass in clients that test regularly. That is expected as lean mass includes hydration as mentioned and your organs, including intestines. So unless you fast and cleanse, which sucks, you will see variations. We explain it is a bit like a stock chart. You will see ups and downs. The trend is what matters.

We see far more consistency in the fat mass measurements.

All that being said…find a method that you are comfortable with and measure your success. If you are in Denver…check us out.

Hydration can effect it but if you are overhydrated it can ovee
er estimate %bf. Also remember generally the percent fat I have seen in my study thus far is is it overestimating but remember it takes into account anadroid and gynoid fat which could result in a higher percentage.

From what I can tell, you can cheat the Dexa Scan Process by consuming mass amounts of water just prior to testing. Your weight may go up, but water is lean, so your percentage of lean mass will increase, lowering your body fat% No?

Currently, conflicting opinions exist as to whether the DEXA is considered the “gold standard” in body composition assessment. Initially, DEXA was created to estimate bone mineral density, which it does accurately. It is still unclear whether the DEXA can account for differences in FFM due to hydration status (Rougbenoff, Kehayias, Dawson- Hughes, & Heymsfield, 1993). The DEXA assumes that hydration of FFM remains constant at 73%. If an individual is hyperhydrated, the DEXA may overestimate FM which could result in a larger %BF (Kelly, Berger, & Richardson, 1998; Pietrobelli, et al., 1999). These results suggest that in order to get accurate estimates of %BF, individuals must continue their regular patterns of hydration or results may be affected. Additionally, all of these methods are subjective to error because none of the proposed methods have been directly measured against cadavers (Brunton, Bayley, & Atkinson, 1993; Toombs, Ducher, Shepherd, & De Souza, 2012).

I have been training ( bodybuilding ) for a couple of years and decided to have a dexa scan to determine my body fat %. My first result gave me a reading of 8.7% with total 6550g fat mass. After 4 weeks of very intense dieting I decided to have another dexa scan. This time I went to a different clinic with a different machine. My result was 4.2% with total 3090g fat mass. The result somewhat surprised me as I wasn’t expecting it to be that low. Visually, the change in my physique has been very noticeable.

Is it possible to drop that much body fat over 4 weeks ? Is anyone sceptical on the accuracy of the Dexa scan? Do results vary between machines?

I validated the Body Metrix Ultrasound machine (US) while comparing it to the DEXA and Bodpod. I measured Fat Mass Fat Free Mass and Percent Body Fat. My results where as followed. There was a significant difference in between all machines when measuring percent body fat, no significant differences between fat free mass between machines and there was a significant different in fat mass between the DEXA to US, and DEXA to Bodpod for Fat Mass. When the Ultrasound was compared to the Bodpod it seemed to be reliable, in fact with in 1-2 percent of the Bodpod, however when it came to comparing it to the DEXA which is considered the Gold Standard, when measuring percent body fat it varied by alot. This has to do with the assumptions made for a 2 compartment method i.e. the Bodpod and Ultrasound compared to a three compartment method such as the DEXA. I only used 85 healthy college-aged subjects, the Ultrasound machine has no been validated in other population, so whether it accurate in other populations is unknown. It has been shown to be accurate when measuring %BF, Fat Mass, and Fat Free mass, when compared to the Bodpod however, if it is accurate when looking at changes over time in different populations in unknown. This research is vitally important especially when a lot of the body composition is very expensive where this new ultrasound is inexpensive or relatively cheaper when compared to other known and validated methods to estimate body composition.

I did a Bod Pod and a DEXA scan within a month of each other and got two completely different results. Bod Pod says I’m at 25%, and the DEXA scan says I’m at 42%. DEXA scan is completely BS measuring body fat.

A DXA Scan is absolutely not bullshit!!!!!!!! You must have gone to a facility that did not calibrate the machine. There is a company out of Chicago and Atlanta that uses it correctly. Try them you will not be let down.

I had a dexa scan show 15.5% and a water test show 29% within two days of each other. The scan results better matches up to my small size and high level of muscle definition. Not sure what to think? I work out 2 hours a day and am very lean so the water results that put me at an ‘average’ level seemed weird.

INTRODUCTION: Obesity has become a worldwide epidemic and is a major contributor to both cardiovascular disease and Type II diabetes. In result, there has been an increase in new technology being introduced into the field of body composition assessment. Recently, a new ultrasound (US) device was created to estimate body composition. However, to date, only one study exists comparing this device to other established and validated body composition assessment methods. PURPOSE: The purpose of this study was to investigate the validity of an Ultrasound System (US), and compare it to Dual Energy X-Ray Absorptiometry (DEXA) and Air Displacement Plethysmography (ADP) in estimating Percent Body Fat (%BF), Fat Free Mass (FFM), and Fat Mass (FM) in college-aged students. METHODS: Eighty-four participants volunteered for this study. All participants were instructed to adhere to the following pretest protocol: 1) No food or drink 2 hours prior 2) Avoid strenuous exercise 24 hours prior to the test 3) Wear tight fitting clothing (i.e. lycra or spandex material). Body composition was assessed using the US, DEXA, and ADP in which all measurements obtained were performed in accordance with manufacture’s guidelines. One-way repeated measures analysis of variance with Bonferroni adjustments to compare differences in %BF, FFM and FM between DEXA to ADP, DEXA to US, and US to ADP. The significance level for all comparisons was set to p < 0.05. RESULTS: The DEXA was greater (25.5 ± 8.5%) vs US (21.6 ± 6.29%) and ADP (19.4 ± 7.5%), p < 0.001, when measuring %BF and the US was greater (21.7 ± 6.6%) vs ADP (19.9 ± 7.9%), p < 0.03, when measuring %BF. The DEXA was greater (37.35 ± 14.97 pounds) vs US (39.38 ± 11.9 pounds) and ADP (30.58 ± 16.18 pounds), p < .000 when measuring FM. However, there was no significant difference in FM between US vs ADP.CONCLUSION: This study demonstrated significant differences between the DEXA, US and ADP when estimating %BF, and FM in college-aged students. This may be due to the measurement methodology, between machines. Therefore, further investigation is warranted.

Did you perform a Bland-Altman to measure the agreement between US and ADP with the gold standard of DEXA. This will compare the differences between each measurement for each person. Since you do not know the true value of the variables you are measuring you can determine the agreement between the devices. This provides more information than a repeated measures anova.

Hi,
I have been having alot of problems with the DEXA scan. As an athlete I am forced to use it often to measure body fat and muscle mass. And many times I have been given negative results to my body fat or muscle mass changes. Just about every time I have taken it, it has been at the strongest point of my body and it may say I am down in 7 pounds of muscle and up 3 pounds of fat. Alot of times I say how can this be when I am stronger in the weight room than ever, and also faster and in my best shape. It can be sometimes discouraging when I am constantly working hard to only improve. Over time i have started to believe a lot of the information it gives me cannot be accurate. I really don’t know what to do about it, since it is one of the uses to measure success. Do you have any advice?

I am doing a secondary data analysis from a large parent study with renal transplant patients. I cannot find any information that quantifies abdominal fat by DEXA scan.If I use a ratio of trunk fat to the sum of left & right leg fat, would that be accurate?
I know the total body fat percentages that are considered overweight and obese, but I don’t know what amount of trunk fat is considered abdominal obesity. If a person has total fat of 28% and trunk fat of 6779.6g what calculation would I use to determine abdominal v lower body adiposity? ( this dexa shows right leg 3210.3 g of fat and left leg 3332.2g of fat)
If you have any resources that you can point me to, I would greatly appreciate it.
Cathy

“Old” studies? Some of the studies referenced are only 5 years old or so.

If you are going to comment on my blog, then I expect you to provide scientific evidence to support your comment, complete with scientific references. What you have provided is not “honest truth”…you have provided random data, with no reference where it came from. It is also apparent that you do not understand that CT scans are no more a gold standard for determining body comp than DEXA is. So your CT scan/DEXA comparison is not valid.

I would like to ask if body position during a DEXA scan can affect results. I am very tall, 195cm, and lean 56 year old with almost zero subcutaneous fat. I am very active working out 3 or more days/week. I recently had a DEXA examine and because of my height, I was instructed to lie in the fetal position rather than supine.

My results indicated 16,732 grams of fat (37 pounds) on my 195 pound frame. That is the equivalent of over 4.5 gallon jugs of fat the vast majority of which would have to be in my gut. Since my abdominals are flat, I can’t see how there’s room for all that plus my internal organs. Then my bone density is 2.008 g/cm2 which is almost off the chart for any age.

Could there be an issue with my body position during the scan or possibly operator error in conducting the scan?

See if you can get this article. It should answer your question. The issue is there is not set guidelines for the DEXA! Techniques for Undertaking Dual-Energy X-Ray
Absorptiometry Whole-Body Scans to Estimate Body
Composition in Tall and/or Broad Subjects

My DEXA tech said for very tall individuals, they will either skip the head, or the feet.

Skipping the feet eliminates less mass from the test.
If testing more than once, he will test the feet on a 2nd or 3rd test to get the data for the feet, after the head data has already been inputted into the system.

testing in a fetal position is crazy. It would throw off all the testing as the person is supposed to be testing lying down on their back. doing anything else would throw off the diagnostic software.

My recent scan indicated my body fat to be 24% which seems very high considering I work out daily, intense strength training twice a week with very heavy weights. The scan did indicate less than 1 lb of visceral fat so all the fat is subcutaneous and it seemed to be spread all over my body fairly evenly. My diet is extremely clean and consistent. The one thing I thought could throw the results off would be a smart lipo procedure I had abut 3-4 months prior on a couple of trouble spots I could not address with diet or exercise. Only about 1 1/4 liters of fat were removed but as a result, the trauma to the tissue may have caused water retention that has also increased my weight by 5-7 pounds overnight. Could water retention in the tissue and fat in these areas result in an overstated BF%?

No, the only thing water retention could do is UNDER estimate body fat.
24% body fat isn’t that bad for a female
It is not the amount of sport you do that determines your weight/fat, but the daily netto calories you eat.

I agree with Ralph. 24% is considered “fit” for a female, anything under 32% is healthy (unless it is too low i.e. less than 13%).
You are probably thinking about subcutaneous fat, you need to account that this machine measures everything, your brain is about 20% fat, fat is marbled through out muscles, fat is in our abdominal cavity, in our organs and females have breasts, mainly fat tissue

I will participate in a research that includes body composition (ow and obesity) in children (mostly 0-36 mo), but also for other studies we will include older children (comparisons between groups but also longitudinal measurements since it would be a cohort study). We would like to buy the best equipment for this purpose but we are still not sure if the DEXA or BodPod. I thought BodPod was not that good for young infants and DEXA not for changes in time. So, what would you recommend? and please if you have some references from your papers I would appreciated it.

If your using 0-36 months. I might recommend a Peapod from Cosmed if your using children 0-36 months. The issue with the Bodpod for children is stature. However, the do have a booster seat for the Bodpod now. See links below. As far as DXA being used. I know it is not good for tracking changes over time. However, this will only be good if the software that is used is for pediatrics and changes overtime to account for differences as children age would be best. In my professional opinion, I feel that deuterium oxide dilution (D20) may be your option to track changes overtime. This is due to children’s total body water changes too much overtime and the other devices mentioned may not be as accurate and are effected by hydration. For example, the Bodpod assumes that were 73.8% water however, we know this to differ with both children and older individuals. Additionally, the DXA also assumes we are at 73.8% so children may violate these assumptions. With that said I have never seen a longitudinal study with children that was more than 6 months long. This is the type of research I want to do as a career and think its vital to know whether these body composition devices can actually longitudinally track changes overtime in children, healthy individuals, overweight, obese, and the elderly.

If you have further questions or want to talk more about it. My email is kjohnson4@unm.edu

DEXA is ideal for changes over time but not so much so for repeated testing i.e. daily.
DEXA is a medical device used in hospitals including on children. You just need to make sure the normative data and correct software is on the machine

I work in radiology, and I’ve done dexa scans with brand new 2014 machines, and they are nearly spot on. It depends on the body fat location for the machine. It will go by percentages of each body part and make a total assessment. It is about 1%-2% off usually for water weight changes but there is nothing you can really do about that.

Dexa gets two things absolutely correct: absolute body fat weight and skeleton weight. This is all I would want to know, because I just want to know how much fat I can realistically lose from point A to point B without going, say, under 8% of my total weight for my total fat weight.

Your ‘LBM’ will change always with hydration and stool. This would scew the percentage of body fat but that really does not bother me so much: the absolute fat weight can be utilized.

Just for the record Ralph DEXA does not get body fat mass absolutely correct. It measures bone mineral content very accurately because that is what they were designed to do. Then it estimates lean body mass based on the volume of lean mass it scans. Then finally it estimates body fat mass by subtracting the bone mineral content and lean body mass from your total weight. It does not specifically measure fat. None of the tools do. Hence the fluctuations between methods because of variations in bone density and muscle hydration levels.

Understandably, there is no perfect machine. However, if you take two test within a short time period and the results are relatively close you should know your rough stats with the Dexa Machine. I had a Dexa Scan twenty days apart recently. I lifted really heavy almost everyday in between. According to the scan I gained 1.8lbs in lean mass. I lost about 2.5lbs overall. About 30 days later (which is today), I’m about 7lbs lighter and a bit stronger, so i want to go back soon. My Z-Score bounced from 2.4 to 2.7 in 20 days (which is a standard deviation of about 1 in 200 people for bone density), and the Dexa is very accurate with bone. The bone increase bolsters my claim that the Dexa variation isn’t a result of machine inconsistency but rather hard gym work. I really believe SO MANY PEOPLE are in denial about how much body fat they have. I was a little surprised myself at my results, but accept my results as pretty accurate.
My 2 results below (5’8″ Male , age 38):

Thinks about this. My lean mass appears higher than average for someone of my profile. I weighed 187 (now 180, about 30 days later just for the record). I held 187 very well with zero gut sticking out and just some small love handles. Yet, I was rocking over 50lbs of fat on my last test. I don’t think people are really appreciating how absolutely low 15% body fat is for a male or 25% for a female. In order for me to be 15% body fat, using my lean mass of 136lbs (add lean mass and bone mineral content) , I would have weigh 160lbs to be 15% body fat. Seems reasonable for a male at 5’8″ at first glance, but that would be low for me. In college, I was absolutely shredded and weighed 170lbs, I got a body fat reading with calipers (which i do not believe for 1 second in retrospect) of only 7%. I could assure you that at 160lbs I would easily fit size 30 designer jeans, which is pretty darn skinny. Dexa may not be perfect, but lets not be in denial. It is the pretty accurate especially if you get a few test to make sure a mistake wasn’t made or you weren’t’ moving during the scan. Accept that the numbers aren’t perfect but accept that numbers are not wildly off by large percentages. This article was written a while ago,and technology is a beast. Currently, our sceintific knowlege doubles its capiticty every 7-8 years. That means we learned more from 2007-2015, then we learned from 2007 all way back to 200,000 years ago at the rough start of Home Sapiens. Insane if you really wrap your head around it. That is relevant because the Dexa Machines are only getting better and better like everything else at a very rapid pace. Technology analysis from 2, 3, 4 years ago is often not relevant to today’s environment. No fault of the author of course. Time just marches on at ever increasing speed of technological advancements. So embrace your Dexa readings. Try a couple of scans close together or perhaps a couple different Dexa machines. Most people are not going to see meaningful fluctuations. The more sample size you have the more confident you can be in the results.

Individualization!!!!!
I don’t quite understand body fat %. I a weight lifter, have been for 25 years. I’ve been told I have 25% body fat, that means I have almost 50 lbs of fat on me. I’m 5’5″ and weigh 193 lbs. You can see all the muscle definition in my chest arms legs and back. I do have some fat on the waste. I’m 40 yrs old can can still bench 365lbs. If I’m carrying 48 lbs of fat, where is it. At most I’m carrying 15-20 lbs of fat. That puts me close to 10%. I just don’t get it. If a man is taller how does that make him have less body fat. A good picture for you to paint of me is this, I’m a 6’3″ man stuffed in a 5’5″ body. Lots of dense muscle and bone from years of heavy wight training.

it’s quite common, your weight and size could actually be very low but you could have low level lean mass. You didn’t waste your money but you probably didn’t have your results explained correctly. Bare in mind women have breasts, mainly fat tissue

The 770 or 720? As long as it is a Multi Frequency BIA then they appear to be accurate. However, this means you must keep the guidelines consistent. These include not eating, drinking, exercising, four hours prior to testing. You also must stay normally hydrated.

Hello. I had a DEXA recently and it said my body fat was 39% (I’m male). This is very strange because, although I do have a bit of a belly, my BMI is normal, and my belly width is less than half my height.
According to the internet (I know…), 39% is far into the obese category, but I do not look like the people in the pictures to represent even 30%, never mind almost 40. Based on the pictures I’m around 25%.
And, if body fat is often measured by pinchers – that’s only subcutaneous fat, isn’t it? If I’m 39% it must all be intra-abdominal. How could pinchers possibly detect that?
So here is a question: could DEXA scan results be affected by prosthetics? I have both ceramic and silicon prosthetics in my body. Could their presence skew the results?
Many thanks for your opinion

John. The dexa is correct within 1-2% of your actual body fat.
If you “have a bit of a belly”, then you will be above 30% BF and more likely in the 35-40% range. The dexa of 39% is probably correct.

one year ago, I was 5’9.5″, approx 190lbs, waist size 36.5″, and estimated BF% of 37%.
two months ago, I was 187lbs, significantly more muscle, waist size down to 34″, DEXA scans at 26%, bodpod 28.9%.

What I do agree with you, is the internet categories are way off in terms of descriptives. They were likely formulated 40-50 years ago before the current obesity epidemic.

DEXA scan would not be affected by prosthetics. the two-component scans would be.

calipers test sub-cutaneous fat, but fat is laid down in equal measure, sub-cut and intra-abdominal.
calipers test the sub-cutaneous and can give fairly reliable readings because the %BF scale takes into account that inside the gut.

[…] unrealistic (but more accurate) methods, but I would have liked him to have pointed out that these aren’t very reliable or consistent either, just to stem the increasing number of people from going out and attempting to get these tests […]

[…] This is pretty much the formula that my Facebook chum Gregory O’Gallagher, at the ripe old age of 27, has used to conclude that he has hit his genetic limit. Poor Greg, he really needs to reel his neck in (and trust DEXA scans less). […]

[…] Comparison with some other measurement device that was also inaccurate but believe that to be an infallible measurement tool. (People often refuse to accept that even DEXA scans for example, can be off by a few percentage points.) […]

[…] of what is achievable naturally for my height.“. He really needs to reel his neck in (and trust DEXA scans less) as he’s doing himself a grave injustice here by placing limiting beliefs on himself, which […]